Recent data that mutations in genes encoding dynein components cause Amyotrophic Lateral Sclerosis indicate that dynein function and retrograde axonal transport are required for motor neuron survival. However, the critical molecules transported by dynein motors are not known. Neurotrophins, peptide growth factors released by targets of innervation, initiate a survival signal that must be propagated through the axon. Cell death in Amyotrophic Lateral Sclerosis (ALS), then, may result from impaired transport of a long-range neurotrophin survival signal. In the proposed studies, we will test the hypothesis that long distance signaling by neurotrophins requires intracellular transport processes that are aberrant in ALS. In support of this hypothesis, we have found that dynein function within axons is required for survival of neurons that depend on target-derived neurotrophic factors. In the proposed experiments we will investigate the nature of the intracellular trafficking responsible for retrograde transport of Trk receptors. We will determine whether mutations that cause ALS in humans impede neurotrophin retrograde signaling. These experiments will provide insight into the pathogenesis of ALS, and identify potential therapeutic targets. We have three specific aims: Aim 1: To test the hypothesis that neurodegeneration in ALS results from the loss of long range neurotrophin signaling. To determine whether genetic changes that predispose to motor neuron degeneration impair retrograde neurotrophin signaling, we will use SOD-1 mice (transgenics expressing the SOD-1 G93A mutation). We will ask whether retrograde neurotrophin signaling is impaired in the neurons of these animals fated to develop ALS. Aim 2: To determine whether Alsin, the product of the ALS2 gene at 2q33-35, is required for vesicular trafficking of Trk receptors and for retrograde signaling by neurotrophins. Mutations in ALS2 have been implicated in a large number of familial motor neuron disorders. Alsin, the protein encoded by ALS2, is a guanine nucleotide exchange factor for the endosomal protein Rab5, and facilitates endocytic sorting in the nervous system. We will determine whether impaired Alsin function results in a loss of neurotrophin survival signals. Aim 3: To identify the nature of the dynein motors that transport long range survival signals, and whether these are assembled in response to neurotrophin stimulation. The composition of dynein motor complexes varies based on the cargo that is transported. We will take both a candidate approach and a proteomic approach to identify components of the motor that transports activated Trks, and whether these motors are regulated by neurotrophins. Taken together, these studies will determine how defects in axonal transport cause selective degeneration of motor neurons. The identification of survival signals that depend on transport, and the transport mechanisms, will lead to new therapeutic approaches to ALS.